I think.

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What is it now? Bad or good? I might have to empty out my fridge!

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I prefer to think the reason is more related to the gradual loss of oxygen on the planet. Animals over a certain mass need exponential amounts of oxygen to survive. As the atmosphere thinned, larger animals would have gotten smaller to compensate.

I realize I have nothing to back this up, and instead would like to posit that they were killed by disease.

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Herbivores, on the other hand, will often substitute a less preferred plant for their usual fare when forced to do so.

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As we've seen from the eruptions of volcanoes like Mount Pinatubo and Krakatoa, while large amounts of ash can be dumped into the air and cause major shifts in light levels and temperature, the suspended particulates do not remain indefinitely, and the turbulence of the atmosphere would certainly wash out the particulates on a fairly short timespan, say less than 10 years. Damage would definitely done, though I think it's safe to say that given the violence of the blow, a great deal of material might have been ejected into space. It's hard to say how much material would have stayed in the atmosphere, and what kind of coverage it would generate. Certainly not all plant or animal life would have died.

There would have been a fairly fast die-off of the larger plant-eating dinosaurs, as they required tons of vegetation daily to survive. Smaller dinosaurs would have survived on the remaining vegetation, though that too would slowly disappear to the extent that it would not support the remaining plant-eaters. Meat-eaters would thrive on the carrion, but eventually even those sources would dry up, and they would end up turning on themselves. Disease would run rampant. All the while, out tiny ancestors would be riding this out.

And yes, this would take time. 300,000 is not inconceivable. You've got to remember that natural selection is at work as well, so some species are better equipped to adapt to the sudden changes in conditions.

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The problem is that without years spent learning about a field of study, you have no clue whether or not what you're applying is actually logic. And to be honest, it really doesn't sound like you know much about anything here. 300 000 years for an entire biosphere to die out with the assumption that a majority of plants are largely wiped out in a matter of a few years? That's just silly.

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